The present invention relates to an apparatus which is useful for rapidly dispensing precise quantities of a viscous liquid, e.g., solder paste, in exact locations. In the past, numerous such dispensing devices have been described in various publications. However, none of the devices to date has been effective at dispensing particularly small quantities or "shots" of solder paste, e.g., about 0.5 cc, or dispensing these shots in a particularly rapid fashion, e.g., up to about 3 shots-per-second.
Solder paste presents a particular challenge to those skilled in the dispensing art due to its non-homogenous nature, and due to its mixture of miniature solder solids (typically metal particles of about 40 micron diameter size) which are suspended in a solution of flux, binders, solvent and other components.
Most modern dispensers are known to place a drop of solder in the desired location through the use of computer-controlled robot systems which accurately position the dispenser head and activate the dispenser to eject a precise amount of solder paste.
Among the types of devices used in the past are air-driven syringes, peristaltic pumps, gear displacement arrangements and positive-displacement pumps.
Air driven syringes usually rely upon a syringe having a fine diameter tip to dispense the shot. The syringe is pressurized intermittently, usually to about 50 PSI for up to three seconds. This forces the shot of solder paste out of the metal dispensing tip onto the board surface. Unfortunately, this method of dispensing solder paste is rather slow, and pressurizing the solder paste tends to cause the particulate solids, flux and binders to separate. Under pressure, the more fluid components contained therein tend to separate and flow more readily, leaving behind some of the particulate metal. Separation of the paste in this fashion tends to clog the dispensing tip, since the particulate metal spheres may melt under high pressure, forming a metal mass which is too large to pass through the fine diameter of the dispensing tip.
The peristaltic pump type of solder paste dispensing device seeks to avoid this separation hazard by employing a low pressure system with a relatively stiff piece of tubing, usually comprised of plastic, which is clamped at each end. The upper clamp is opened, allowing the tube volume to fill with solder paste. The upper clamp is then tightened, and the lower clamp is released. The upper clamp is then simply slid down the tubing, causing a shot of solder to be squeezed out of the lower opening of the tube. However, peristaltic pumps do not dispense precise quantities of solder paste, since the volume of the tube changes with repeat usage. Eventually the tube must be replaced since the tube wall tends to develop cracks.
The positive displacement pump is in many ways the most effective present method of dispensing a precise amount of solder. The pump tends to apply the least amount of stress to the solder paste, thereby avoiding the separation of the solder paste into its components. Moreover, positive displacement to date has given the most precise shot sizes, since the volume of the displacement rod remains constant. However, positive displacement by itself is unacceptable at high speeds, particularly when the viscosity of the solder paste is low, which allows paste to drip out of the dispensing tip.
Due to this dripping, a variable amount of solder may be dispensed. Increasing pressure in the standard positive displacement system may cause the solder to separate into its component parts, thereby leading to jamming or melting of the metal particles which results in clogging as previously described.
One hand held solder paste dispenser manufactured by Techcon Systems, Inc., a division of Creative Automation, currently on the market uses a positive displacement pump to deliver shot sizes as small as about 0.024 inches in diameter. The dispensing tip incorporated therein relies upon a Luer-lock system, and is approximately 4 cms long. The dispenser requires positive pressure of around 10 psi to deliver solder shots. About 3 shots per second can be delivered.
A similar automated device, manufactured by Creative Automation, uses a positive pressure pump to deliver shot sizes about the same size as the manual dispenser described above.
A positive pressure dispensing apparatus is described in U.S. Pat. No. 3,806,084, issued to Seese and entitled "Improved Valve Dispensing Apparatus." The Seese patent describes a dispensing apparatus comprising a housing defining a tubular chamber which communicates with a material inlet and exit port, and a unitary valve member defining a passageway. The valve member is slidable in the chamber. The Seese patent describes a wiping action on the chamber wall by the valve member during its return stage, which causes a suction force on undispensed material, thereby preventing the solder from dripping out of the exit port.
U.S. Pat. No. 4,095,722 issued to Miller on June 20, 1978, and entitled "Dripless Dispenser and Methods of Dispensing a Flowable Material", describes a related positive pressure dispenser, using a bore and a pinch-off tube fitted within the bore. The solder paste reservoir is connected to one end of the pinch-off tube, and at least one dispensing nozzle or needle is removably coupled to the other end of the tube. The pinch-off tube is squeezed to dispense a shot of solder. Release of the squeezed pinch-off tube causes the residual solder paste in the pinch-off tube to be drawn back away from the dispensing tip. This reverse flow of solder paste ("snuff-back"), is useful for preventing dripping. However, until the present invention, snuff-back was not available in combination with the physical obstruction of the solder paste reservoir tube using the side wall of the dispensing rod, thereby enhancing shot size control and placement accuracy.
U.S. Pat. No. 4,294,381, issued to Miller on Oct. 13, 1981, and entitled "Dispenser for and a Method of Dispensing a Material from a Non-Pressurized Material Reservoir" discloses a solder paste dispenser having a non-pressurized reservoir with a bore therethrough. The reservoir is connected to one end of a pinchoff tube, while at least one nozzle or needle is attached to the other end of the tube. As above, the tube is squeezed to force solder paste out of the needle, and released to draw any residual solder paste away from the dispensing tip to effect snuff-back. While this snuff-back feature is useful, it may not completely prevent solder paste drippage when particularly low viscosity solder paste is dispensed. Consequently, one object and advantage of the present invention is to combine the benefits and preferred features of positive displacement dispensers with a syringe-type dispenser.
Another object and advantage of the present invention is to maximally enhance and supplement the snuff-back of the residua solder paste through the use of a dispensing rod, the side wall of which occludes the reservoir tube when in its "down", dispensing position, thereby more precisely regulating shot size and facilitating shot dispensing speed and placement. The present invention uses a very short positive pressure flow distance e.g., 2 cm., minimizing the separation of solids.